1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting and receiving system information related to the configuration of a frame in a Time Division Duplexing (TDD) wireless communication system.
2. Description of the Related Art
Wireless communication systems, which provided voice-driven services in their early stages, have developed into high-speed, high-quality wireless packet data communication systems to provide data services and multimedia services. As these wireless communication systems, a variety of wireless communication systems, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and Long Term Evolution (LTE) proposed by 3rd Generation Partnership Project (3GPP), including the Code Division Multiple Access (CDMA) network that was providing 2nd Generation (2G) mobile communication services in the past, have recently been developed to support high-speed, high-quality wireless packet data transmission services.
The LTE system is a typical high-speed packet data system, to which multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) is applied in a Downlink (DL) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) is applied in an Uplink (UL). The LTE system may have the configuration of Frequency Division Duplexing (FDD) and TDD systems. In the FDD system, a DL and a UL use different frequency bands for their transmission, and an evolved Node B (eNB) (or a Base Station (BS) in the CDMA system) and a User Equipment (UE) (or a Mobile Station (MS) in the CDMA system) perform transmission and reception at the same time. In the TDD system, a DL and a UL use the same frequency band for their transmission, and an eNB and a UE cannot perform transmission and reception at the same time. Therefore, the TDD system uses a method in which a UE and an eNB agree on their transmission and reception times in advance and transmit data at predetermined times.
The FDD system has two frequencies, and uses one of them for a frequency band for DL transmission and the other one for a frequency band for UL transmission. The TDD system has only one frequency band, divides the one frequency band in the time axis, and performs DL transmission at arbitrary times and UL transmission at other times.
In the TDD system, UL transmission and DL transmission are performed in accordance with predetermined rules. The LTE system has a total of, for example, 7 kinds of TDD radio frame configurations (hereinafter, referred to as ‘TDD configurations’ for short), and once determined by the system, the TDD configurations are not likely to be changed. In the case of TDD, if cells have different frame configurations, their transmission/reception may fail due to interference. Therefore, all cells in a specific area have the same TDD configuration and are synchronized with each other, so UL and DL transmissions may be performed at the same time.
In the LTE system, both TDD and FDD sub-frames have a length of 1 ms in the time axis, and have the full-LTE transmission bandwidth (BW) in the frequency axis. A sub-frame is divided into two slots along the time axis. In the LTE system, a transmission bandwidth includes a plurality of Resource Blocks (RBs), and each RB is used as a basic unit for resource allocation. Each RB may include 12 tones arranged in the frequency axis and 14 OFDM symbols arranged in the time axis. This sub-frame includes a control channel region for carrying control channels and a data channel region for carrying data channels. A Reference Signal (RS) for channel estimation is inserted into the control channel region and the data channel region.
The LTE system, to which TDD is applied, may not cope with the dynamically changing amount of UL/DL data, because once determined, the TDD configuration may not be changed in a short period of time. In other words, even though a large amount of UL data is generated for a certain period of time, the system may not use a DL sub-frame in a UL interval, for UL transmission. Many studies have been made to solve these problems.
As described above, the existing wireless communication system, to which TDD is applied, may not transmit Uplink (UL) data using a Downlink (DL) sub-frame even if a large amount of UL data is generated in a UL interval. This situation may occur frequently in a wireless communication environment where a large number of cells are hierarchically configured.
FIG. 1 illustrates an example of a conventional wireless communication system In FIG. 1 the wireless communication system has a communication environment in which macro cells and pico cells are arranged hierarchically, and the wireless communication system performs TDD communication.
Referring to FIG. 1, reference numeral 101 represents macro cells, and reference numeral 102 represents pico cells. The pico cells 102 generally transmit data with less power, compared to the macro cells 101, and are installed in the area(s) where there is a request for a large amount of data traffic from the macro cells 101. The area where there is a request for a large amount of data traffic may mean that the request for data traffic may significantly and dynamically change with the passage of time. For example, if a plurality of users perform DL data reception and Voice-over Internet Protocol (VoIP) transmission/reception, a UE needs to transmit a specific amount of UL data while receiving a large amount of DL data.
Therefore, the communication system illustrated in FIG. 1 may configure a TDD radio frame (hereinafter, referred to as a ‘TDD frame’ for short) so as to use most sub-frames for a DL and use a smaller number of sub-frames for UL transmission. However, if a UE needs to transmit a large amount of data to an eNB at a specific time or if there is a large amount of UL VoIP data to transmit, the UE may instantly request a large amount of UL resources from the eNB. The common TDD system, however, may not appropriately accept this UE's request for UL resources, because the common TDD system is designed such that eNBs connected to the same network may have the same TDD configuration, in order to prevent interference caused by collision of UL transmission and DL transmission between an eNB and a UE, or between neighbor eNBs. Furthermore, in the TDD system, each eNB needs to independently change its TDD configuration in order to adaptively cope with the variable amount of data traffic, which causes interference, making it impossible to actually transmit data.
FIG. 2 illustrates an example of a frame configuration in a conventional TDD-LTE system.
Referring to FIG. 2, generally, in the TDD-LTE system, one radio frame 201 has a length of 10 ms, and includes two half radio frames 202 each having a length of 5 ms. One half radio frame 202 includes 5 sub-frames 203. In other words, one radio frame 201 includes 10 sub-frames 203, and each sub-frame 203 has a length of 1 ms. The 10 sub-frames 203 may have various combinations of sub-frames for DL and UL transmission in accordance with 7 kinds of TDD configurations supported by the LTE system. An example of the TDD configurations is illustrated in Table 1 below. The TDD configurations in Table 1 are merely illustrative, and in the LTE system, the TDD configurations may have a variety of different forms.
TABLE 1Downlink-Uplink-to-UplinkdownlinkSwitch-config-pointSubframe NumberurationPeriodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
Referring to Table 1 for a TDD configuration #0 , its sub-frame indexes #0 and #5 are represented as ‘D’ and used for DL transmission, and its sub-frame indexes #2, #3, #4, #7, #8 and #9 are represented as ‘U’ and used for UL transmission. In the TDD configuration #0 , its sub-frame indexes #1 and #6 are represented as ‘S,’ which indicates a special sub-frame 205. The special sub-frame 205 includes 3 regions of a DL pilot time slot (DwPTS), a guard period (GP), and an UL pilot time slot (UpPTS). The special sub-frame 205 functions as a switching point between a DL and a UL in TDD.
Due to its relative short UL interval, the special sub-frame 205 is used only for transmission of Physical Random Access CHannel (PRACH) and Sounding Reference Signal (SRS), and is not used for transmission of data or control channels. The GP is designed to ensure the time that an eNB needs in order to receive UL signals while transmitting DL signals.
As to TDD configuration information, Table 1 illustrates TDD configurations #0 to #6, each of which has a sub-frame(s) used to perform the same transmission at all times regardless of the TDD configuration. In the example of Table 1, the sub-frames #0, #1, #2 and #5 have the same sub-frame configuration at all times regardless of the kinds of TDD configurations. The sub-frames #3, #4, #6 and #7 may have different sub-frame configurations depending on the TDD configuration (i.e., depending on the kind of TDD configuration).
In a conventional LTE system, once a TDD configuration is determined (for example, one of the TDD configurations #0 to #6 in Table 1 is determined or selected), the determined TDD configuration is not changed by the amount of data traffic, and if the TDD configuration is changed in disregard of TDD interference from neighbor cells, it takes a time of about 640 ms or more to change the TDD configuration. The time of 640 ms is the time it takes for a UE to update system information by receiving system change information transmitted by an eNB. A TDD system that can change the TDD configuration due to a change in system information in this way will be referred to as a dynamic TDD system.
UEs used in the conventional TDD-LTE system may be classified into a UE supporting the dynamic TDD system and a UE that does not support the dynamic TDD system. The UE that does not support the dynamic TDD system may not recognize or detect a change in TDD configuration, which is caused by a change in system information. Therefore, the UE that does not support the dynamic TDD system may not achieve UL data transmission and DL data reception due to the changed TDD configuration, and also may not determine whether the eNB operates the TDD configuration for a DL or for a UL, at the time that the UE performs channel measurement in a DL sub-frame. As a result, in the dynamic TDD system, the eNB may not manage the UE that does not support the dynamic TDD system.
FIG. 3 illustrates an example of changing a TDD configuration using system information in a conventional TDD-LTE system.
Referring to FIG. 3, reference numeral 301 represents a frame interval, to which a TDD configuration A is applied, and reference numeral 303 represents a frame interval, to which a TDD configuration B is applied. In FIG. 3, the frame interval 301, to which the TDD configuration A is applied, has a relatively large amount of UL data, while the frame interval 303, to which the TDD configuration B is applied, has a relatively large amount of DL data. For UL and/or DL transmission, a UE and an eNB use frames 305 to which the TDD configuration A is applied, in the interval 301, and use frames 307 to which the TDD configuration B is applied, in the interval 303.
If it is expected that the amount of DL data will be greater than the amount of UL data, while using the frames 305 to which the TDD configuration A is applied, the eNB transmits system information to the UE at a time 309, the system information including TDD configuration information that is changed to change the TDD configuration from the TDD configuration A to the TDD configuration B. The UE receives the system information including the changed TDD configuration information, and changes the TDD configuration accordingly. In this case, since the changed TDD configuration has UL/DL transmission timing different from that of the previous TDD configuration, all UEs, which have received the changed TDD configuration information at the time 309, stop the transmission to which the TDD configuration A is applied, and start transmission to which the TDD configuration B is applied, depending on the changed TDD configuration information.
FIG. 4 illustrates a method for changing system information including TDD configuration information in a conventional TDD-LTE system.
Referring to FIG. 4, an interval which is shaded, as represented by reference numeral 401, is an interval where System Information Block 1 (SIB1) in system information is transmitted, and an interval which is hatched as shown by reference numeral 403 is an interval where SIB is transmitted, whose transmission period is different from that of SIB1. Reference numeral 405 represents a modification period (or change period) of the system information.
In the conventional LTE system, the system information that an eNB transmits to a UE, includes a System Information Block (SIB). With respect to the SIB, there are a plurality of different SIBs starting from, for example, SIB1, and the SIBs include different system information. The SIBs are different from each other in terms of the transmission time and period. SIB1 is transmitted at intervals of 80 ms, and retransmitted at intervals of 20 ms. For the other SIBs, the system determines their transmission time at intervals of a multiple of 80 ms. Upon receiving system information from the eNB, the UE maintains the received system information until the system information is changed.
If there is a change/update in the system information, the UE may receive a system information change indicator or SIB change indicator 407 over a paging channel 409 transmitted by the eNB. The SIB change indicator 407 may be transmitted as, for example, single-bit information. The SIB change indicator 407 indicates the presence of a change in system information if its value is ‘1’, and indicates the absence of a change in system information if its value is ‘0’. In the example of FIG. 4, the UE may determine whether the system information is changed or not, based on the SIB change indicator 407 received from the paging channel 409. Upon detecting the presence of a change in system information based on the paging channel 409, the UE receives, for example, a changed SIB1 in a modification period 411 for system information 413, which follows the modification period in which the UE received the paging channel 409, and then sequentially changes the rest of the system information, if necessary.
In the conventional LTE system, TDD configuration information is included in SIB1 information. If an eNB changes TDD configuration information, the eNB indicates a change in system information using a paging channel 409 in the same way, and a UE receives a changed SIB1 415 in the next system information modification period, and may receive changed TDD configuration information 417 from the changed SIB1 415. The SIB1 415 has a change tag (not shown) for system information, and the change tag may be set such that, for example, 31 changes are possible in a period of 3 hours. Based on the change in the change/update tag, the UE may detect a change in system information. The system information may be changed once in every system information modification period 411 in the fastest case, but due to the constraints that a total of 31 changes are possible in 3 hours, the system information may be changed once every 5 minutes on average. The UE updates 419 the previously received SIB1 using the changed SIB1 415.
However, in the LTE system, the above-described existing system information changing method is limited to be applied to the dynamic TDD system, and all UEs need to change the system information. However, since the existing UE that does not support the dynamic TDD system cannot detect the change in system information, an error may occur between the TDD configuration changed by the eNB and the TDD configuration used by the UE, making the data transmission difficult. In addition, if an error occurs during reception of system information, this problem becomes more serious.